1. Field of the Invention
The invention relates to a method for preparing dielectric ceramic compositions having a formula represented by Ba(Zn1/3Ta2/3)O3 for use in a filter of a base station for mobile and satellite communication, and more particularly a method for preparing dielectric ceramic compositions having a formula represented by Ba(Zn1/3Ta2/3)O3 (hereinafter referred as BZT dielectrics or BZT dielectric compositions), which have a dielectric constant of at least 28.5, a Q value of at least 200,000, and a stable resonant frequency temperature coefficient of xc2x12.
2. Description of the Related Art
The high frequency dielectrics are essential material forming parts used in a duplexer and a band-pass filter for a wireless telephone or a terminal of mobile communication, radio LAN, satellite broadcasting, etc. However, there are demands for development of high frequency dielectrics desirably to have a high dielectric constant, a high Q value, and a temperature coefficient of near 0.
Typical dielectric substances having a high Q value and a high dielectric constant include complex perovskite dielectrics such as BZT, etc. BMT has a high Q value of about 200,000, however it has a dielectric constant lower than 30 of BZT. BZT is known to have a dielectric constant of 30 or more as well as a high Q value and a resonant frequency temperature coefficient of near 0. Therefore, it attracts public attention as promising dielectrics.
However, BZT has disadvantages in that it has difficulty in production since Zn volatilizes in a sintering process. Further, good dielectric properties cannot be attained. In order to solve such problems, conventionally three kinds of methods described below were used.
The first method comprises a step of sintering pure BZT at a high temperature of 1,400xc2x0 C. to 1,550xc2x0 C. for at least 60 hours. However, there is difficulty in sintering for such a long time. Moreover, although the dielectric constant and resonant frequency temperature coefficient of the produced dielectrics are good, the Q value is low as 100,000. Therefore, the BZT cannot be made of use in practice.
The second method comprises forming a solid solution of BZT and BaZrO3. According to this method, it is possible to reduce the sintering time within 10 hours or less depending on composition of BaZrO3. However, the produced dielectrics have a relatively low Q value of 150,000 although it has a good dielectric constant and resonant frequency temperature coefficient. Therefore, it is also difficult to use this method in practice.
The final method comprises forming a solid solution of BZT and (Ba1-xSrx) (Ga1/2Ta1/2)O3. The dielectrics produced according to this method have good dielectric properties such as a dielectric constant of 29 to 30, a resonant frequency temperature coefficient of xc2x15 and a Q value of 200,000 or more. However, the method requires an additional thermal treatment at a temperature of 1,450xc2x0 C. for a period of long time of 24 hours more after the sintering step at a temperature 1,550xc2x0 C. for 2 hours. Thus, this method cannot be made of used in practice.
Therefore, the present invention has been made in order to solve the above-described problems involved in the methods for preparing BZT dielectric ceramic compositions. Now, we have discovered that by adding a metal oxide containing 3+ or 4+ metal ion, which is smaller than or equal to Zn in their sizes, the problems in association with the volatilization of Zn in the process for producing BZT can be overcome.
An object of the present invention is to provide a method for preparing BZT dielectric ceramic compositions, which have good dielectric properties and can be used as material forming a filter for a base station of mobile/satellite communication.
In order to achieve the above object, the present invention provides a method for preparing high frequency Ba(Zn1/3Ta2/3)O3 dielectric ceramic composition, which comprises the steps of: mixing BaCO3, Ta2O3 and ZnO powder and calcining to thereby obtain Ba(Zn1/3Ta2/3)O3 powder; adding one of oxide selected from a group consisting of Al2O3, Ga2O3, ZrO2, TiO2, and SnO2 powder to the Ba(Zn1/3Ta2/3)O3 powder to thereby obtain a powder mixture; and sintering the mixture obtained from the above step.
The feature of the present invention resides in adding one of metal oxide containing 3+ or 4+ ion such as Al2O3, Ga2O3, ZrO2, TiO2, and SnO2 powder in a proper amount on the base of the total composition so that the sintering can be effected within a short time at a temperature of 1,500xc2x0 C. to 1,550xc2x0 C. provide BZT composition having a high Q value, a high dielectric constant and resonant frequency temperature coefficient of near 0.
Particularly, according to the present invention, it is possible to control the resonant frequency temperature coefficient and dielectric constant by selecting the additive oxide. Further, the sintering time can be reduced and BZT dielectric ceramic composition having good dielectric properties can be prepared.
In accordance with the present invention, firstly BaCO3, Ta2O3 and ZnO were weighed and mixed together. Then, the resulting mixture was sintered at a temperature of 1,200xc2x0 C. for 2 hours to synthesize into a BZT dielectric ceramic composition.
The synthesized BZT composition powder was ball-milled and dried. One of oxides selected from a group consisting of Al2O3, Ga2O3, ZrO2, TiO2, and SnO2 powder was added.
According to our study, in order to enhance the dielectric properties of the BZT dielectrics, as above-mentioned, the vacant which is generated by volatilization of Zn having a low melting point is to be filled. We discovered that the vacant could be filled with a metal ion having a size smaller than or equal to Zn and a atomic value similar to that of Zn. Zn has an atomic value of 2+ and has a size of 0.08 nm at an ion state. Al has an atomic value of 3+ and a size of 0.057 nm at an ion state, Zr has an atomic value of 4+ and a size of 0.079 nm at an ion state, Ti has an atomic value of 4+ and a size of 0.64 nm at an ion state, and Sn has an atomic value of 4+ and a size of 0.74 nm at an ion state. Therefore, we concluded that ions derived from these atoms would readily substitute for the Zn volatilized.
That is, by adding these metal ions as metal oxide to the dielectric compositions so that they fill the vacant, which is generated by volatilization of Zn having a low melting point, the influence due to the volatilization of Zn can be minimized. Consequently, it is possible to obtain a BZT dielectric with a uniform lattice structure of a high density and sintering characteristics.
The amount of the metal oxides, which is added according to the present invention, is varied on the atomic value of the metal. The metal oxide containing a metal having an atomic value of 3+, such as Al2O3 and Ga2O3, is added preferably in an amount of 0.7 to 1.0 mole %. The metal oxide containing a metal having an atomic value of 4+, such as TiO2, ZrO2 and SnO2, is added preferably in an amount of 1.5 to 2.0 mole %.
After adding the metal oxide, the powder mixture is subjected to a sintering step. Preferably, the sintering temperature is at least 1500xc2x0 C. so that the BZT synthetic powder reacts sufficiently with the metal oxide and exceeds not 1550xc2x0 C. so as to suppress the volatilization of Zn. The sintering time is controlled not to exceed 10 hours considering the excessive volatilization of Zn.